So you need to install something like the 'Generic Webhook Plugin' on your Jenkins server if you've not already, that configures you an End Point to direct your Webhooks at. You then set up a Webhook via the Webhook configuration page (see "Configure a Mainframe Webhook" in the manual).
The Webhook server will then monitor your directory for log files and then consume these and post Webhooks out to your Jenkins server. You can then use the 'Generic Webhook Plugin' to consume these hooks on the Server as part of a Pipeline or Freestyle Project.